Mucocele of the appendix is a rare disease entity which is infrequently diagnosed prior to a laparotomy. With progress in diagnostic procedures, such as the use of ultrasonography, computed tomography, and colonoscopy, preoperative diagnosis of the appendiceal mucocele has become possible. The case that we have experienced involves an eighty five-year old male. He was admitted with bowel obstruction symptoms such as severe abdominal pain, tenderness, and increased bowel sound sounds. He was diagnosed as having appendiceal mucocele by an abdominal CT and colonoscopy, He was operated on and the surgical specimen revealed appendicea] mucocele. (Korean J Gastrointest Endosc 18: 777-780, 1998)